<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="./js/vue.js"></script>
    <style>
        .box{
            width: 100px;
            height: 100px;
            border:1px solid #000
        }
        .over{
            background-color: springgreen;
        }
        .out{
            border-radius: 25px;
        }
    </style>
</head>
<body>
    <div id="root">
        <div :class="list" @mouseover="list.push('over')" @mouseout="list.push('out')"></div>
    </div>
    <script>
        new Vue({
            el:'#root',
            data(){
                return{
                    list:['box']
                }
            }
        })
    </script>
</body>
</html>